The project described in this application addresses a mechanistic study aimed at understanding the mechanism of action of MHC class II accessory protein, HLA-DO and its role in generation of immunodominant epitopes. Immunodominance is a phenomenon that has long been recognized but yet remains unclear to date. It is well known that the immune system focuses on and responds to very few representative epitopes (referred to as immunodominant epitopes) from invading pathogenic insults ranging from such as infectious agents and, antigenic targets in autoimmune diseases, allergy, and cancer. In each all these cases, the immune system either responds positively or fails to respond to antigenic peptides in the context of MHC molecules. Recent advances in our understanding of the antigen presentation pathway have shown that the steps of antigen processing and selection critically influence the peptide repertoire presented to T-cells. Recently, we have made considerable progress in developing a reductionist antigen processing system for MHC class II molecules that utilizes five purified protein components of the class II antigen presentation pathway. Notably, this system yielded physiologically relevant immunodominant epitopes restricted to HLA-DR1. In this proposal, we will extend this MHC II system to include another MHC class II molecules HLA-DO and would explore its contribution to epitope capture and processing. Aim 1 would explore mechanistic aspects of how HLA-DO interacts with MHC II and HLA-DM, leading to regulation of peptide binding, and in Aim 2 we would investigate contributions of HLA-DO to epitope capture and editing from full length protein antigens and immunodominance. A clear understanding of HLA-DO function and its role in antigen processing and the selection of immunodominant epitopes, can guide the design of effective immunotherapeutics.